A long-lived mutant with the rage of a beast and the soul of a Samurai, James "Logan" Howlett's past is filled with blood, war, and betrayal. Possessing an accelerated healing factor, enhanced senses, and bone claws in his hands that, along with his skeleton, are coated in adamantium, Wolverine is the ultimate weapon.

Depends what comic you read. Logically, it's from his mutation. But in both a recent Jeff Loeb arc, as well as in the Earth X (Or was it Planet X) miniseries, it's suggested that Wolverine and other feral characters are all descended from a separate, non-human race of wolf people. It was pretty stupid.

I think it was just growing up tough that made him so animalistic, and conceivably his mutation has, like, strengthened that in recent years. I don't see why Weapon X would bring that out of people. If anything, they'd want him to keep that in check most of the time.